As is well-known, the diffuser for a gas turbine engine converts compressor discharge air dynamic pressure to static pressure to allow air to enter the combustor with minimal combustor section pressure loss. The greater the diffuser pressure recovery, the less is the penalty to the engine cycle. Typically, the diffuser consists of a prediffuser upstream of a dump diffuser, which feeds air into and around an aerodynamically shaped cowl located ahead of the combustor. Though its performance is not ideal, a dump diffuser provides good pressure recovery in a short axial distance, and its performance is insensitive to the velocity profile at the inlet of the prediffuser.
An example of a prediffuser/dump diffuser is disclosed in U.S. Pat. No. 4,272,955 granted to J. S. Hoffman and M. E. Abreu on June 16, 1981 to which reference is made for details of diffuser construction. In this patent, supra, the dump diffuser is designed to include steps located axially in the direction toward the combustor. The steps are provided by including generally squared-off shoulders formed on the outer and inner walls of the diffuser itself. In this arrangement, the wall of the cowl ahead of the combustor is formed in a smooth surface defining an aerodynamically shaped member for providing a smooth transition of the flow into the shrouds of the combustor.
As shown in the U.S. Pat. No. 4,272,955, supra, two sudden expansions, one at the prediffuser exit and the other downstream therefrom adjacent the diffuser outer wall are provided. This dual step arrangement distinguishes over the single large sudden expansion at the prediffuser exit and it has been found that the dual step is more efficacious than the single step of the heretofore known dump diffuser designs.
Other types of dump diffusers consist of a single, large, sudden expansion in flow area immediately downstream of the prediffuser exit.
The current invention replaces the single step expansion of a conventional dump diffuser with multiple steps, at different axial locations; the multiple step arrangement has been found to more effectively convert dynamic pressure to static pressure than do heretofore known dump diffusers. In particular, locating one of the steps on the contoured surfaces of the combustor cowl while reducing the size of the step at the prediffuser exit has been shown to reduce the sudden expansion losses measured in the dump diffuser. Since expansion losses are linked to air velocity upstream of the expansion, the improved performance of the multiple step diffuser is attributable to delaying some of the expansion until velocity is reduced due to the area increase of the first sudden expansion.
Test results from a full-size, 2-D diffuser rig simulating the current diffuser (one-step) geometry or the stepped diffuser geometry reveal that the pressure recovery in the two-step design is higher than that in the one-step design. Likewise, 1-D calculations show that multiple step designs should be superior in pressure recovery. This invention contemplates that the dump diffuser should include at least two steps, one at the prediffuser exit, and the second downstream, adjacent to the cowl. In a design with more than two steps, two of the steps would be those mentioned above.